Blind Decoding

ABSTRACT

A method includes selecting, in a system in which a communication device searches for unscheduled transmissions of downlink control information for said communication device, a combination of search spaces for unscheduled transmissions of downlink control information for a communication device by a number of transmission techniques. The method includes transmitting an indication of the result of the selected combination to said communication device. Apparatus and program products are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims the benefitunder 35 U.S.C. §365(c), of P.C.T. application PCT/EP2012/054669, filedon Mar. 16, 2012, the disclosure of which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

This invention relates generally to wireless networks and, morespecifically, relates to downlink control information in wirelessnetworks.

BACKGROUND

This section is intended to provide a background or context to theinvention disclosed below. The description herein may include conceptsthat could be pursued, but are not necessarily ones that have beenpreviously conceived, implemented or described. Therefore, unlessotherwise explicitly indicated herein, what is described in this sectionis not prior art to the description in this application and is notadmitted to be prior art by inclusion in this section.

Radio communication systems typically involve the transmission ofdownlink control information via radio resources whose precise identityis not known to the intended recipient(s) of the downlink controlinformation. A receiving device finds downlink control informationintended for it by repeatedly selecting different candidate sets ofradio resources and attempting to successfully decode the signalscarried on said sets of radio resources. This is known as blinddecoding.

It has been proposed to make such unscheduled transmissions of downlinkcontrol information according to an enhanced transmission technique bywhich gains, such as frequency-domain packet scheduling (FDPS) gain, canbe targeted; whilst continuing to also make unscheduled transmissionsaccording to the conventional transmission technique.

There has been identified the challenge of reducing the processing powerrequired by a communication device to perform blind decoding under theabove-mentioned proposal.

SUMMARY

This section contains examples of possible implementations and is notmeant to be limiting.

There is hereby provided a method, comprising: in a system in which acommunication device searches for unscheduled transmissions of downlinkcontrol information for said communication device: selecting acombination of search spaces for unscheduled transmissions of downlinkcontrol information for a communication device by a plurality oftransmission techniques: and transmitting an indication of the result ofthe selected combination to said communication device.

According to one embodiment, the method further comprises deciding howmuch search space to assign to unscheduled transmissions of downlinkcontrol information for said communication device by one or more of saidplurality of transmission techniques relative to unscheduledtransmissions of downlink control information for said communicationdevice by one or more others of said plurality of transmissiontechniques.

According to one embodiment, said selecting comprises selecting apredefined combination from a plurality of predefined combinations.

According to one embodiment, said plurality of predefined combinationsincludes a predefined combination according to which no search space isassigned to unscheduled transmissions of downlink control information byone of said plurality of transmission techniques.

According to one embodiment, a plurality of aggregation levels are usedfor the transmission of downlink control information, and a respectivesearch space is predefined for each aggregation level for eachtransmission technique; and the method further comprises: selecting arespective set of one or more aggregation levels for each of saidplurality of transmission techniques; and transmitting an indication ofthe selected respective sets of aggregation levels to said communicationdevice.

According to one embodiment, said selecting is performed at least partlyon the basis of one or more of: (a) the size of the load on the cell inwhich the transmissions are made, and (b) channel state information forthe wireless interface with the communication device.

There is also hereby provided a method, comprising: in a system in whicha communication device searches for unscheduled transmissions ofdownlink control information messages for said communication deviceaccording to a plurality of message formats: selecting a respective setof one or more search spaces for each of said plurality of messageformats; and transmitting to said communication device an indication ofthe result of said selection of respective sets of one or more searchspaces.

According to one embodiment, a plurality of aggregation levels are usedfor the transmission of downlink control messages, and a respectivesearch space is predefined for each aggregation level; and the methodfurther comprises: selecting a respective set of one or more aggregationlevels for each of said plurality of message formats; and transmittingan indication of the selected respective sets of aggregation levels tosaid communication device.

According to one embodiment, the plurality of message formats includesfirst and second message formats providing control information for atransmission according to respective first and second transmissionmodes; and the selecting takes into account an indication of therespective transmission error probability for the first and secondtransmission modes.

There is also hereby provided a method, comprising: in a system in whicha radio access network transmits indications of the results ofselections of combinations of search spaces for unscheduledtransmissions by a plurality of transmission techniques of downlinkcontrol information for one or more communication devices: searching ata first communication device for unscheduled transmissions of downlinkcontrol information for said communication device; wherein saidsearching is performed either: (a) on the basis of a transmittedindication of the result of a selection for said first communicationdevice of a combination of search spaces for unscheduled transmissionsby said plurality of transmission techniques of downlink controlinformation; or (b) in the absence of a transmitted indication of theresult of a selection for said first communication device of acombination of search spaces for unscheduled transmissions by saidplurality of transmission techniques of downlink control information, onthe basis of a default combination of search spaces for unscheduledtransmissions by said plurality of transmission techniques of downlinkcontrol information for said first communication device.

There is also hereby provided a method comprising: searching at a firstcommunication device for unscheduled transmissions of downlink controlinformation for said communication device by a plurality of transmissiontechniques; wherein said searching is performed on the basis of atransmitted indication of the result of a selection of a combination ofsearch spaces for unscheduled transmissions of downlink controlinformation for said first communication device by said plurality oftransmission techniques.

According to one embodiment, said transmitted indication indicates howmuch search space has been assigned to unscheduled transmissions ofdownlink control information for said first communication device by oneor more of said plurality of transmission techniques relative tounscheduled transmissions of downlink control information for said firstcommunication device by one or more others of said plurality oftransmission techniques.

According to one embodiment, said transmitted indication identifies oneof a plurality of predefined combinations.

According to one embodiment, said plurality of predefined combinationsincludes a predefined combination according to which no search space isassigned to unscheduled transmissions of downlink control information byone of said plurality of transmission techniques.

According to one embodiment, a plurality of aggregation levels are usedfor the transmission of downlink control information, and a respectivesearch space is predefined for each aggregation level for eachtransmission technique; and said transmitted indication identifies arespective set of one or more aggregation levels for each of saidplurality of transmission techniques.

There is also hereby provided a method, comprising: in a system in whicha radio access network transmits unscheduled transmissions of downlinkcontrol information messages for a plurality of communication devicesaccording to a plurality of message formats, and in which said radioaccess network transmits indications of the results of selections forone or more communication devices of respective sets of one or moresearch spaces for each of said plurality of message formats: searchingat a first communication device for unscheduled transmissions ofdownlink control information for said first communication device;wherein said searching is performed either: (a) on the basis of atransmitted indication of the result of a selection for said firstcommunication device of a respective set of one or more search spacesfor each of said plurality of message formats; or (b) in the absence ofa transmitted indication of the result of a selection for said firstcommunication device of a respective set of one or more search spacesfor each of said plurality of message formats, on the basis of arespective default set of one or more search spaces for each of saidplurality of message formats.

There is also hereby provided a method, comprising: searching at a firstcommunication device for unscheduled transmissions of downlink controlinformation messages for said first communication device according to aplurality of message formats; wherein said searching is performed basedon a transmitted indication of the result of a selection of a respectiveset of one or more search spaces for each of said plurality of messageformats.

According to one embodiment, a plurality of aggregation levels are usedfor the transmission of downlink control messages, and a respectivesearch space is predefined for each aggregation level; and wherein thetransmitted indication identifies a respective selected set of one ormore aggregation levels for each of said plurality of message formats.

There is also hereby provided an apparatus comprising: a processor andmemory including computer program code, wherein the memory and computerprogram code are configured to, with the processor, cause the apparatusto: in a system in which a communication device searches for unscheduledtransmissions of downlink control information for said communicationdevice: select a combination of search spaces for unscheduledtransmissions of downlink control information for a communication deviceby a plurality of transmission techniques: and transmitting anindication of the result of the selected combination to saidcommunication device.

According to one embodiment, the memory and computer program code arefurther configured to, with the processor, cause the apparatus to:decide how much search space to assign to unscheduled transmissions ofdownlink control information for said communication device by one ormore of said plurality of transmission techniques relative tounscheduled transmissions of downlink control information for saidcommunication device by one or more others of said plurality oftransmission techniques.

According to one embodiment, the memory and computer program code arefurther configured to, with the processor, cause the apparatus to:select a predefined combination from a plurality of predefinedcombinations.

According to one embodiment, said plurality of predefined combinationsincludes a predefined combination according to which no search space isassigned to unscheduled transmissions of downlink control information byone of said plurality of transmission techniques.

According to one embodiment, a plurality of aggregation levels are usedfor the transmission of downlink control information, and a respectivesearch space is predefined for each aggregation level for eachtransmission technique; and wherein the memory and computer program codeare further configured to, with the processor, cause the apparatus to:select a respective set of one or more aggregation levels for each ofsaid plurality of transmission techniques; and transmitting anindication of the selected respective sets of aggregation levels to saidcommunication device.

According to one embodiment, the memory and computer program code arefurther configured to, with the processor, cause the apparatus toperform said selection at least partly on the basis of one or more of:(a) the size of the load on the cell in which the transmissions aremade, and (b) channel state information for the wireless interface withthe communication device.

There is also hereby provided an apparatus comprising: a processor andmemory including computer program code, wherein the memory and computerprogram code are configured to, with the processor, cause the apparatusto: in a system in which a communication device searches for unscheduledtransmissions of downlink control information messages for saidcommunication device according to a plurality of message formats: selecta respective set of one or more search spaces for each of said pluralityof message formats; and transmitting to said communication device anindication of the result of said selection of respective sets of one ormore search spaces.

According to one embodiment, a plurality of aggregation levels are usedfor the transmission of downlink control messages, and a respectivesearch space is predefined for each aggregation level; and wherein thememory and computer program code are further configured to, with theprocessor, cause the apparatus to: select a respective set of one ormore aggregation levels for each of said plurality of message formats;and transmit an indication of the selected respective sets ofaggregation levels to said communication device.

According to one embodiment, the plurality of message formats includesfirst and second message formats providing control information for atransmission according to respective first and second transmissionmodes; and wherein the memory and computer program code are furtherconfigured to, with the processor, cause the apparatus to: perform saidselection taking into account an indication of the respectivetransmission error probability for the first and second transmissionmodes.

There is also hereby provided an apparatus comprising: a processor andmemory including computer program code, wherein the memory and computerprogram code are configured to, with the processor, cause the apparatusto: in a system in which a radio access network transmits indications ofthe results of selections of combinations of search spaces forunscheduled transmissions by a plurality of transmission techniques ofdownlink control information for one or more communication devices:search at a first communication device for unscheduled transmissions ofdownlink control information for said communication device; wherein saidsearching is performed either: (a) on the basis of a transmittedindication of the result of a selection for said first communicationdevice of a combination of search spaces for unscheduled transmissionsby said plurality of transmission techniques of downlink controlinformation; or (b) in the absence of a transmitted indication of theresult of a selection for said first communication device of acombination of search spaces for unscheduled transmissions by saidplurality of transmission techniques of downlink control information, onthe basis of a default combination of search spaces for unscheduledtransmissions by said plurality of transmission techniques of downlinkcontrol information for said first communication device.

There is also hereby provided an apparatus comprising: a processor andmemory including computer program code, wherein the memory and computerprogram code are configured to, with the processor, cause the apparatusto: search at a first communication device for unscheduled transmissionsof downlink control information for said communication device by aplurality of transmission techniques; wherein said searching isperformed on the basis of a transmitted indication of the result of aselection of a combination of search spaces for unscheduledtransmissions of downlink control information for said firstcommunication device by said plurality of transmission techniques.

According to one embodiment, said transmitted indication indicates howmuch search space has been assigned to unscheduled transmissions ofdownlink control information for said first communication device by oneor more of said plurality of transmission techniques relative tounscheduled transmissions of downlink control information for said firstcommunication device by one or more others of said plurality oftransmission techniques.

According to one embodiment, said transmitted indication identifies oneof a plurality of predefined combinations.

According to one embodiment, said plurality of predefined combinationsincludes a predefined combination according to which no search space isassigned to unscheduled transmissions of downlink control information byone of said plurality of transmission techniques.

According to one embodiment, a plurality of aggregation levels are usedfor the transmission of downlink control information, and a respectivesearch space is predefined for each aggregation level for eachtransmission technique; and said transmitted indication identifies arespective set of one or more aggregation levels for each of saidplurality of transmission techniques.

There is also hereby provided an apparatus comprising: a processor andmemory including computer program code, wherein the memory and computerprogram code are configured to, with the processor, cause the apparatusto: in a system in which a radio access network transmits unscheduledtransmissions of downlink control information messages for a pluralityof communication devices according to a plurality of message formats,and in which said radio access network transmits indications of theresults of selections for one or more communication devices ofrespective sets of one or more search spaces for each of said pluralityof message formats: search at a first communication device forunscheduled transmissions of downlink control information for said firstcommunication device; wherein said searching is performed either: (a) onthe basis of a transmitted indication of the result of a selection forsaid first communication device of a respective set of one or moresearch spaces for each of said plurality of message formats; or (b) inthe absence of a transmitted indication of the result of a selection forsaid first communication device of a respective set of one or moresearch spaces for each of said plurality of message formats, on thebasis of a respective default set of one or more search spaces for eachof said plurality of message formats.

There is also hereby provided an apparatus comprising: a processor andmemory including computer program code, wherein the memory and computerprogram code are configured to, with the processor, cause the apparatusto: search at a first communication device for unscheduled transmissionsof downlink control information messages for said first communicationdevice according to a plurality of message formats: wherein saidsearching is performed based on a transmitted indication of the resultof a selection of a respective set of one or more search spaces for eachof said plurality of message formats.

According to one embodiment, a plurality of aggregation levels are usedfor the transmission of downlink control messages, and a respectivesearch space is predefined for each aggregation level; and wherein thetransmitted indication identifies a respective selected set of one ormore aggregation levels for each of said plurality of message formats.

There is also hereby provided a computer program product comprisingprogram code means which when loaded into a computer controls thecomputer to: in a system in which a communication device searches forunscheduled transmissions of downlink control information for saidcommunication device select a combination of search spaces forunscheduled transmissions of downlink control information for acommunication device by a plurality of transmission techniques: andtransmitting an indication of the result of the selected combination tosaid communication device.

There is also hereby provided a computer program product comprisingprogram code means which when loaded into a computer controls thecomputer to: in a system in which a communication device searches forunscheduled transmissions of downlink control information messages forsaid communication device according to a plurality of message formats:select a respective set of one or more search spaces for each of saidplurality of message formats; and transmitting to said communicationdevice an indication of the result of said selection of respective setsof one or more search spaces.

There is also hereby provided a computer program product comprisingprogram code means which when loaded into a computer controls thecomputer to: in a system in which a radio access network transmitsindications of the results of selections of combinations of searchspaces for unscheduled transmissions by a plurality of transmissiontechniques of downlink control information for one or more communicationdevices: search at a first communication device for unscheduledtransmissions of downlink control information for said communicationdevice; wherein said searching is performed either: (a) on the basis ofa transmitted indication of the result of a selection for said firstcommunication device of a combination of search spaces for unscheduledtransmissions by said plurality of transmission techniques of downlinkcontrol information; or (b) in the absence of a transmitted indicationof the result of a selection for said first communication device of acombination of search spaces for unscheduled transmissions by saidplurality of transmission techniques of downlink control information, onthe basis of a default combination of search spaces for unscheduledtransmissions by said plurality of transmission techniques of downlinkcontrol information for said first communication device.

There is also hereby provided a computer program product comprisingprogram code means which when loaded into a computer controls thecomputer to: search at a first communication device for unscheduledtransmissions of downlink control information for said communicationdevice by a plurality of transmission techniques; wherein said searchingis performed on the basis of a transmitted indication of the result of aselection of a combination of search spaces for unscheduledtransmissions of downlink control information for said firstcommunication device by said plurality of transmission techniques.

There is also hereby provided a computer program product comprisingprogram code means which when loaded into a computer controls thecomputer to: in a system in which a radio access network transmitsunscheduled transmissions of downlink control information messages for aplurality of communication devices according to a plurality of messageformats, and in which said radio access network transmits indications ofthe results of selections for one or more communication devices ofrespective sets of one or more search spaces for each of said pluralityof message formats: search at a first communication device forunscheduled transmissions of downlink control information for said firstcommunication device: wherein said searching is performed either: (a) onthe basis of a transmitted indication of the result of a selection forsaid first communication device of a respective set of one or moresearch spaces for each of said plurality of message formats; or (b) inthe absence of a transmitted indication of the result of a selection forsaid first communication device of a respective set of one or moresearch spaces for each of said plurality of message formats, on thebasis of a respective default set of one or more search spaces for eachof said plurality of message formats.

There is also hereby provided a computer program product comprisingprogram code means which when loaded into a computer controls thecomputer to: search at a first communication device for unscheduledtransmissions of downlink control information messages for said firstcommunication device according to a plurality of message formats:wherein said searching is performed based on a transmitted indication ofthe result of a selection of a respective set of one or more searchspaces for each of said plurality of message formats.

According to one embodiment, said plurality of transmission techniquesinclude: a first transmission technique involving the transmission ofone or more reference signals specific to said communication device; anda second transmission technique that does not involve transmission ofone or more reference signals specific to said communication device.

According to one embodiment, said plurality of transmission techniquesinclude: a first transmission technique involving the transmission ofboth one or more reference signals specific to said communication deviceand one or more channel-state-information reference signals; and asecond transmission technique involving transmission of one or morecell-specific reference signals.

According to one embodiment, said plurality of transmission techniquesinclude: a first transmission technique involving the precoding andtransmission of demodulation reference signals together with theprecoding and transmission of data symbols defining downlink controlinformation, and the transmission of one or more channel stateinformation reference signals together with the transmission of precodeddata symbols defining downlink control information; and a secondtransmission technique that involves transmission of cell-specificreference signals together with transmission of precoded data symbolsdefining downlink control information.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described in detail hereunder,by way of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 illustrates an example of a cellular network in which embodimentsof the present invention are implemented;

FIG. 2 illustrates an example of apparatus for use at user equipment inFIG. 1;

FIG. 3 illustrates an example of apparatus for use at eNodeB in FIG. 1;

FIG. 4 illustrates an example of a transmission technique fortransmitting downlink control information;

FIG. 5 illustrates an example of another transmission technique fortransmitting downlink control information;

FIG. 6 illustrates an example of operations at a network access node anduser equipment of FIG. 1 in accordance with a first embodiment of thepresent invention;

FIG. 7 illustrates an example of operations at a network access node anduser equipment of FIG. 1 in accordance with a second embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described in detail below, by way ofexample only, in the context of a cellular network operating inaccordance with an E-UTRAN standard.

FIG. 1 illustrates an example of a cellular network in which embodimentsof the present invention can be implemented. The cellular networkincludes cells 4 with transceivers at respective eNodeBs (eNBs). Onlynine cells are shown in FIG. 1, but a mobile telecommunication networkwill typically comprise tens of thousands of cells. Each eNB 2 isconnected by a wired link to a core network (not shown).

FIG. 2 shows a schematic view of an example of user equipment 8 that maybe used for communicating with the eNBs 2 of FIG. 1 via a wirelessinterface. The user equipment (UE) 8 may be used for various tasks suchas making and receiving phone calls, for receiving and sending data fromand to a data network and for experiencing, for example, multimedia orother content.

The UE 8 may be any device capable of at least sending or receivingradio signals to or from the eNBs 2 of FIG. 1. Non-limiting examplesinclude a mobile station (MS), a portable computer provided with awireless interface card or other wireless interface facility, personaldata assistant (PDA) provided with wireless communication capabilities,or any combinations of these or the like. The UE 8 may communicate viaan appropriate radio interface arrangement of the UE 8. The interfacearrangement may be provided for example by means of a radio part andassociated antenna arrangement. The antenna arrangement may be arrangedinternally or externally to the UE 8.

The UE 8 may be provided with at least one data processing entity 203and at least one memory or data storage entity 217 for use in tasks itis designed to perform. The data processor 213 and memory 217 may beprovided on an appropriate circuit board 219 and/or in chipsets.

The user may control the operation of the UE 8 by means of a suitableuser interface such as key pad 201, voice commands, touch sensitivescreen or pad, combinations thereof or the like. A display 215, aspeaker and a microphone may also be provided. Furthermore, the UE 8 maycomprise appropriate connectors (either wired or wireless) to otherdevices and/or for connecting external accessories, for examplehands-free equipment, thereto.

FIG. 3 shows an example of apparatus for use at the eNBs 2 of FIG. 1 andfor serving the cell 4 in which UE 8 is located. The apparatus comprisesa radio frequency antenna array 301 configured to receive and transmitradio frequency signals; radio frequency interface circuitry 303configured to interface the radio frequency signals received andtransmitted by the antenna 301 and the data processor 306. The radiofrequency interface circuitry 303 may also be known as a transceiver.The apparatus also comprises an interface 309 via which it can send andreceive information to and from one or more other network nodes. Thedata processor 306 is configured to process signals from the radiofrequency interface circuitry 303, control the radio frequency interfacecircuitry 303 to generate suitable RF signals to communicate informationto the UE 6 via the wireless communications link, and also to exchangeinformation with other network nodes via the interface 309. The memory307 is used for storing data, parameters and instructions for use by thedata processor 306.

It would be appreciated that the apparatus shown in each of FIGS. 2 and3 described above may comprise further elements which are not directlyinvolved with the embodiments of the invention described hereafter.

As indicated at 3GPP TS 36.212 Section 5.3.3, downlink controlinformation (DCI) can comprise one or more of downlink or uplinkscheduling information, requests for aperiodic CQI (channel qualityinformation) reports, notifications of MCCH (multicast control channel)change and uplink power control commands.

Different message formats are used depending on the purpose of thedownlink control information.

For example, the format identified at 3GPP TS 36.212 Section 53.3.1.2 asDCI Format 1 is used for communicating resource assignments for singlecodeword PDSCH transmissions; and the format identified at 3GPP TS36.212 Section 5.3.3.1.3 as DCI Format 2 is used for communicatingresource assignments for PDSCH for closed loop MIMO operation.

eNB 2 determines a PDCCH format to be transmitted to UE 8, and creates aset of information bits according to the rules set out at 3GPP TS 36.212Section 5.3.3. The PDCCH formats differ according to the number ofcontrol channel elements (CCEs) used for the transmission of the DCImessage, which number is referred to as the aggregation level, L. Forexample, the aggregation level could be 1, 2, 4 or 8.

The information bits comprising one downlink control information (DCI)message are then subjected to the attachment of a CRC (cyclic redundancycheck). The CRC is masked with an RNTI (radio network temporaryidentifier) either associated with the UE 8 for which the DCI message isintended, or with an RNTI associated with the usage of the PDCCH in thecase of a PDCCH intended for multiple recipients.

The resulting set of bits are then subjected to channel coding and ratematching, as described at 3GPP TS 36.212 Section 5.3.3.

As described at 3GPP TS 36.211 Section 6.3, the resulting set of codedbits are transformed into a baseband signal by the following steps: (i)scrambling of coded bits; (ii) modulation of scrambled bits to generatecomplex-valued modulation symbols; (iii) mapping of the complex-valuedmodulation symbols onto one or several transmission layers; (iv)precoding of the complex-valued modulation symbols on each layer fortransmission on the antenna ports; (v) mapping of complex-valuedmodulation symbols for each antenna port to resource elements; and (vi)generation of complex-valued time-domain OFDM signal for each antennaport.

The eNB 2 transmits DCI messages for UE 8 according to two differenttransmission techniques.

The first transmission technique is illustrated in FIG. 4 and is acodebook-based transmission scheme that involves transmitting thebaseband signal defining the DCI message together with cell-specificreference signals (CRS), as described at Section 6.0.1 of 3GPP TS 36.211V10.40. Precoding at eNB 2 and deprecoding at UE 8 is carried out usinga precoding matrix (one of a plurality of predefined different matriceswhose collection is referred to as a codebook), whose identity iscommunicated by eNB 2 to UE 8.

The second transmission technique is illustrated in FIG. 5 and involvessubjecting UE-specific reference signals (DM-RS) (as described atSection 6.10.3 of 3GPP TS36.21) to the same precoding as thecomplex-valued modulation symbols derived from the DCI bits; andtransmitting the baseband signal derived from the DCI message togetherwith CSI reference signals (CSI-RS) (as described at Section 6.10.5 of3GPP TS 36.211) and the precoded UE-specific reference signals. Thisnon-codebook-based transmission technique does not use predefinedprecoding matrices for the precoding and deprecoding, eNB 2 freelyselects downlink transmission weights for each antenna of the antennaarray without being restricted to one of a limited predefined number ofcombinations. UE 8 makes an estimate of the overall channel between eNB2 and UE 8 using the UE-specific reference signals that are precoded inthe same way as the set of complex-valued modulation symbols derivedfrom the DCI message. PDCCHs transmitted according to this enhancedtransmission technique are known as enhanced PDCCHs (ePDCCHS).

UE 8 is not aware of exactly which sets of radio resources (controlchannel elements (CCEs) in a sub-frame) carry the baseband signaldefining the DCI message(s) for UE 8. In order to find PDCCH(s) intendedfor UE 8, UE 8 performs blind decoding by repeatedly selecting differentsets of radio resources (that it knows are used for PDCCHs/ePDCCHs) asPDCCH/ePDCCH candidates, and attempting to successfully decode therespective baseband signals. The attempts at decoding involve demaskingthe CRC using the Cell-RNTI assigned to UE 8. If no CRC error isdetected, the decoding attempt is deemed to be successful, and UE 8reads the control information defined by the decoded bits.

Because the decoding attempts involve setting the size of the DCImessage as an input (which size depends on the DCI Format), UE 8 mayneed to subject each PDCCH/ePDCCH candidate to a separate decodingattempt for each of the two DCI formats that are specified for UE 8according to the transmission mode of UE 8 and whether or not thePDCCH/ePDCCH for UE 8 is included in the common search space (CSS) orthe UE-specific search space (USS).

One example of possible aggregation levels for a DCI message is shown inTable 1 below for both PDCCH and ePDCCH. Table 1 also shows the searchspace size for each aggregation level. The search space size indicatesthe size of the set of radio resources (expressed in terms of the numberof CCEs) in which the PDCCH or e-PDCCH of the respective aggregationlevel could be found. The search spaces for each aggregation level arepredefined sets of radio resources (CCEs) whose identity is pre-storedat UE 8. The maximum number of PDCCH/ePDCCH candidates that UE 8 wouldneed to check in order to find a PDCCH/ePDCCH of a certain type (i.e.either PDCCH or ePDCCH) and aggregation level is given by the respectivesearch space size divided by the respective aggregation level. If theDCI message can have one of two DCI formats, UE 8 may need to make up totwo decoding attempts for each PDCCH/ePDCCH candidate, because of thedifferent sizes of the different DCI formats.

Search space S_(k) ^((L)) Number Search space S_(k) ^((L)) Number ofPDCCH of of ePDCCH of Aggre- PDCCH Aggre- ePDCCH gation Size [incandidates gation Size [in candidates Type level L CCEs] M^((L)) level LeCCEs] M^((L)) UE- 1 6 A 1 6 U specific 2 12 B 2 12 V 4 8 C 4 8 W 8 16 D8 16 X Common 4 16 E 4 16 Y 8 16 F 8 16 Z

If no restrictions were set by eNB 2 on what aggregation level are usedfor DCI messages for UE 8, the maximum number of PDCCH/ePDCCH candidatesthat UE 8 would have to make decoding attempts for would be forty-four(44).

According to this first embodiment of the present invention: in order toassist UE 8 in finding the baseband signals for the DCI message(s) forUE 8 in a sub-frame (and thereby reduce the processing power required atUE 8 for blind decoding), eNB 2 sets restrictions on which aggregationlevels (PDCCH formats) it uses for DCI messages for UE 8, and transmitsan indication of the restrictions to UE 8 by higher layer signallingsuch as RRC signalling or MAC-CE signalling, in advance of the PDCCHtransmissions to which the restrictions apply.

With reference to FIG. 6, eNB 2 decides how much total search space toassign to DCI by PDCCH relative to ePDCCH (STEP 602), and selects acombination of predefined search spaces for transmission of DCI by PDCCHand ePDCCH (STEP 604). eNB 2 then transmits to UE 8 a messageidentifying the selected combination of search spaces (STEP 606). UE 8receives this transmitted message (STEP 608), and restricts thesearching it performs as part of bind decoding to the selectedcombination of search spaces identified in the message received from eNB2 (STEP 610).

According to one example: subject to any limitations resulting from theblind decoding capability of UE 8 (i.e. the maximum number of decodingattempts that UE can make in a sub-frame), eNB freely sets each of A toF and U to Z of Table 1 to zero or the maximum number of candidatesaccording to whether or not the respective combination of PDCCH/ePDCCHtype and aggregation level is to be used for UE 8; and information ofthe chosen settings is communicated to UE 8. For example, eNB 2 couldset each of A to F and U to Z such that the maximum total number ofPDCCH/ePDCCH candidates is 22, which is the maximum total number ofPDCCH candidates under the conventional procedure (using only PDCCH andnot ePDCCH) described at Section 9.1.1 of 3GPP TS 36.213. Thiscommunication to UE 8 of values for A to F and U to Z can be done byseparated encoding or joint encoding.

One combination of A to F and U to Z can be chosen as a defaultcombination whose selection does not need to be communicated to UE 8,because UE 8 is preconfigured to adopt this default configuration in theabsence of any transmitted indication about the selection of acombination of A to F and U to Z for UE 8. One example of a defaultcombination is: A=6; B=6; C=2; D=2; E=4; F=2 and all of U to Z=0.According to this default combination, no unscheduled transmissions ofDCI are made via ePDCCH, only via PDCCH.

According to another example aimed at reducing the overhead for thecommunication to UE 8, eNB 2 selects for UE 8 one of a plurality ofpredefined configurations, and communicates the identity of the selectedpredefined configuration to UE 8 by means of a policy numberrecognisable to UE 8 as associated with a combination of values for A toF and U to Z pre-stored at UE 8.

For example, the plurality of predefined configurations could beformulated such that there are predefined configurations with differentamounts of resources allocated to PDCCH and ePDCCH for UE 8. Examplesinclude: (i) a configuration in which A to F are each set to the maximumnumber of candidates and U to Z are each set to zero (i.e. a maximumamount of resources is allocated to PDCCH for UE 8 and no resources areallocated to ePDCCH for UE 8); (ii) a configuration in which the sum ofA to F is 14 and the sum of U to Z is 8 (i.e. about twice as manyresources are allocated to PDCCH for UE 8 as to ePDCCH for UE 8); (iii)a configuration in which the sum of A to F is 8 and the sum of U to Z is14 (i.e. about twice as many resources are allocated to ePDCCH for UE 8as to PDCCH for UE 8); and (iv) a configuration in which A to F are allset to zero and U to Z are each set to the maximum number of candidates(i.e. a maximum amount of resources is allocated to ePDCCH for UE 8 andno resources are allocated to PDCCH for UE 8).

The above-described technique provides a way to tradeoff gain and loadbetween PDCCH and ePDCCH. Allocating more resources to PDCCH and less toePDCCH can help to reduce the conflicting ratio. On the other hand,allocating more resources to ePDCCH and less to PDCCH can increase theFDPS gain, user selective gain etc. eNB 2 decides on an optimumallocation of resources between PDCCH and ePDCCH for UE 8 based on anumber of factors such as load and the state of the channel between eNB2 and UE 8.

According to a second embodiment: the amount of processing powerrequired for blind decoding at UE 8 is reduced by eNB 2 placing furtherrestrictions on the DCI formats that it uses for DCI messages to UE 8 atone or more aggregation levels via PDCCH and/or ePDCCH; andcommunicating the restrictions to UE 8 by higher layer signalling suchas RRC signalling or MAC-CE signalling. For example, by restricting DCImessages including PDSCH scheduling information for UE 8 at one or moreaggregation levels to only one DCI Format instead of two DCI formats,and communicating this restriction from eNB 2 to UE 8, the processingpower required at UE 8 for blind decoding can be reduced. For example,where DC1 Format 2 is used to provide control information required for ascheduled transmission according to a preferred transmission mode, andDCI Format 1 is used to provide control information required for thesame scheduled transmission according to a fall-back transmission mode(which is more reliable than said preferred mode in some situations suchas when the UE is relatively distant from eNB 2); then eNB 2 can decideto reduce the number of aggregation levels at which it sends DCImessages according to DCI Format 2 if it determines that the probabilityof the preferred transmission mode resulting in a successfultransmission falls below a threshold value.

With reference to FIG. 7, eNB 2 decides how much total search space toassign to DCI Format 1 relative to DCI Format 2 (STEP 702), and selectsa combination of predefined search spaces for DCI Formats 1 and 2 (STEP704). eNB 2 then transmits to UE 8 a message identifying the selectedcombination of search spaces (STEP 706). UE 8 receives this transmittedmessage (STEP 708), and restricts the searching it performs as part ofblind decoding to the selected combination of search spaces identifiedin the message received from eNB 2 (STEP 710).

According to a third embodiment, the technique according to the secondembodiment is used in combination with the technique according to thefirst embodiment.

The above-described operations may require data processing in thevarious entities. The data processing may be provided by means of one ormore data processors. Similarly various entities described in the aboveembodiments may be implemented within a single or a plurality of dataprocessing entities and/or data processors. Appropriately adaptedcomputer program code product may be used for implementing theembodiments, when loaded to a computer. The program code product forproviding the operation may be stored on and provided by means of acarrier medium such as a carrier disc, card or tape. A possibility is todownload the program code product via a data network. Implementation maybe provided with appropriate software in a server.

For example the embodiments of the invention may be implemented as achipset, in other words a series of integrated circuits communicatingamong each other. The chipset may comprise microprocessors arranged torun code, application specific integrated circuits (ASICs), orprogrammable digital signal processors for performing the operationsdescribed above.

Embodiments of the invention may be practiced in various components suchas integrated circuit modules. The design of integrated circuits is byand large a highly automated process. Complex and powerful softwaretools are available for converting a logic level design into asemiconductor circuit design ready to be etched and formed on asemiconductor substrate.

Programs, such as those provided by Synopsys, Inc. of Mountain View,Calif. and Cadence Design, of San Jose, Calif. automatically routeconductors and locate components on a semiconductor chip using wellestablished rules of design as well as libraries of pre stored designmodules. Once the design for a semiconductor circuit has been completed,the resultant design, in a standardized electronic format (e.g., Opus,GDSII, or the like) may be transmitted to a semiconductor fabricationfacility or “fab” for fabrication.

In addition to the modifications explicitly mentioned above, it will beevident to a person skilled in the art that various other modificationsof the described embodiment may be made within the scope of theinvention.

What is claimed is:
 1. A method, comprising: selecting, in a system inwhich a communication device searches for unscheduled transmissions ofdownlink control information for said communication device, acombination of search spaces for unscheduled transmissions of downlinkcontrol information for a communication device by a plurality oftransmission techniques: and transmitting an indication of the result ofthe selected combination to said communication device.
 2. A methodaccording to claim 1, comprising deciding how much search space toassign to unscheduled transmissions of downlink control information forsaid communication device by one or more of said plurality oftransmission techniques relative to unscheduled transmissions ofdownlink control information for said communication device by one ormore others of said plurality of transmission techniques.
 3. A methodaccording to claim 1, wherein said selecting comprises selecting apredefined combination from a plurality of predefined combinations.
 4. Amethod according to claim 3, wherein said plurality of predefinedcombinations includes a predefined combination according to which nosearch space is assigned to unscheduled transmissions of downlinkcontrol information by one of said plurality of transmission techniques.5. A method according to claim 1, wherein a plurality of aggregationlevels are used for the transmission of downlink control information,and a respective search space is predefined for each aggregation levelfor each transmission technique; and wherein the method comprises:selecting a respective set of one or more aggregation levels for each ofsaid plurality of transmission techniques; and transmitting anindication of the selected respective sets of aggregation levels to saidcommunication device.
 6. A method according to claim 1, wherein saidselecting is performed at least partly on the basis of one or more of:(a) the size of the load on the cell in which the transmissions aremade, and (b) channel state information for the wireless interface withthe communication device.
 7. A method according to claim 1, wherein saidplurality of transmission techniques include: a first transmissiontechnique involving the transmission of one or more reference signalsspecific to said communication device; and a second transmissiontechnique that does not involve transmission of one or more referencesignals specific to said communication device.
 8. A method according toclaim 1, wherein said plurality of transmission techniques include: afirst transmission technique involving the transmission of both one ormore reference signals specific to said communication device and one ormore channel-state-information reference signals; and a secondtransmission technique involving transmission of one or morecell-specific reference signals.
 9. A method according to claim 1,wherein said plurality of transmission techniques include: a firsttransmission technique involving the precoding and transmission ofdemodulation reference signals together with the precoding andtransmission of data symbols defining downlink control information, andthe transmission of one or more channel state information referencesignals together with the transmission of precoded data symbols definingdownlink control information; and a second transmission technique thatinvolves transmission of cell-specific reference signals together withtransmission of precoded data symbols defining downlink controlinformation.
 10. A method, comprising: in a system in which acommunication device searches for unscheduled transmissions of downlinkcontrol information messages for said communication device according toa plurality of message formats: selecting a respective set of one ormore search spaces for each of said plurality of message formats; andtransmitting to said communication device an indication of the result ofsaid selection of respective sets of one or more search spaces.
 11. Amethod according to claim 10, wherein a plurality of aggregation levelsare used for the transmission of downlink control messages, and arespective search space is predefined for each aggregation level; andwherein the method comprises: selecting a respective set of one or moreaggregation levels for each of said plurality of message formats; andtransmitting an indication of the selected respective sets ofaggregation levels to said communication device.
 12. A method accordingto claim 11, wherein the plurality of message formats includes first andsecond message formats providing control information for a transmissionaccording to respective first and second transmission modes; and whereinthe selecting takes into account an indication of the respectivetransmission error probability for the first and second transmissionmodes.
 13. An apparatus comprising: a processor and memory includingcomputer program code, wherein the memory and computer program code areconfigured to, with the processor, cause the apparatus to: select, in asystem in which a communication device searches for unscheduledtransmissions of downlink control information for said communicationdevice, a combination of search spaces for unscheduled transmissions ofdownlink control information for a communication device by a pluralityof transmission techniques: and transmit an indication of the result ofthe selected combination to said communication device.
 14. An apparatusaccording to claim 13, wherein the memory and computer program code arefurther configured to, with the processor, cause the apparatus to:decide how much search space to assign to unscheduled transmissions ofdownlink control information for said communication device by one ormore of said plurality of transmission techniques relative tounscheduled transmissions of downlink control information for saidcommunication device by one or more others of said plurality oftransmission techniques.
 15. An apparatus according to claim 13, whereinthe memory and computer program code are further configured to, with theprocessor, cause the apparatus to: select a predefined combination froma plurality of predefined combinations.
 16. An apparatus according toclaim 15, wherein said plurality of predefined combinations includes apredefined combination according to which no search space is assigned tounscheduled transmissions of downlink control information by one of saidplurality of transmission techniques.
 17. An apparatus according toclaim 13, wherein a plurality of aggregation levels are used for thetransmission of downlink control information, and a respective searchspace is predefined for each aggregation level for each transmissiontechnique; and wherein the memory and computer program code are furtherconfigured to, with the processor, cause the apparatus to: select arespective set of one or more aggregation levels for each of saidplurality of transmission techniques; and transmitting an indication ofthe selected respective sets of aggregation levels to said communicationdevice.
 18. An apparatus according to claim 13, wherein the memory andcomputer program code are further configured to, with the processor,cause the apparatus to: perform said selection at least partly on thebasis of one or more of: (a) the size of the load on the cell in whichthe transmissions are made, and (b) channel state information for thewireless interface with the communication device.
 19. An apparatusaccording to claim 13, wherein said plurality of transmission techniquesinclude: a first transmission technique involving the transmission ofone or more reference signals specific to said communication device; anda second transmission technique that does not involve transmission ofone or more reference signals specific to said communication device. 20.An apparatus according to claim 13, wherein said plurality oftransmission techniques include: a first transmission techniqueinvolving the transmission of both one or more reference signalsspecific to said communication device and one or morechannel-state-information reference signals; and a second transmissiontechnique involving transmission of one or more cell-specific referencesignals.